Subtle Bite-Angle Influences on N2S2Ni

Inorg. Chem. 2001, 40, 3601-3605

3601

Subtle Bite-Angle Influences on N2S2Ni Complexes Jason J. Smee, Matthew L. Miller, Craig A. Grapperhaus, Joseph H. Reibenspies, and Marcetta Y. Darensbourg* Department of Chemistry, Texas A&M University, College Station, Texas 77843 ReceiVed August 1, 2000

A new N2S2Ni complex based on the 1,4-diazacycloheptane (dach) framework allows the study of the effects of ring size, in fused diamines, on the structural and chemical properties of nickel(II) dithiolate and dithioether complexes. Compared to its 1,5-diazacyclooctane (daco) derivatives, the dithiolate complex (bmedach)Ni and the S-templated, macrocyclic dithioether complex (propyl-bmedach)NiBr2 show decreased cavity sizes with narrower ∠N-Ni-N angles (by ca. 6°) and wider ∠S-Ni-S angles (also by ca. 6°). The electrochemical properties of the dithiolate complexes based on dach and daco are nearly identical, while the (propyl-bmedach)NiBr2 complex shows a 140 mV destabilization of the NiI oxidation state relative to its daco analogue. Molecular structures for the (bmedach)Ni and (propyl-bmedach)NiBr2 complexes and their respective electrochemical and spectroscopic properties are reported.

Scheme 1

Introduction A broad series of ligands based on the diazacyclooctane (daco) framework derivatized with episulfides have been useful for delineating square-planar nickel(II) coordination chemistry.1-3 The study of these complexes has led to a better understanding of the scope of nickel-bound, thiolate-sulfur modification and how such changes influence the redox accessibility of the NiI and NiIII oxidation states, in particular as they might pertain to the redox properties measured in [NiFe]hydrogenase.1 It was verified that the NiI oxidation state is stabilized upon conversion of the thiolates to thioethers (ca. +600 mV per alkylation),1 to RSOx by oxygenation (by ca. +150 mV per O atom),3 and on their interaction with metal cations.2 This stabilization is due to the relief of the Spπ-Nidπ antibonding interaction in the NiSR HOMO,4-6 as well as the increase in the positive charge on the complex. In contrast, the NiIII oxidation state is destabilized in the cationic thioether derivatives, but gains stability when those derivatives contain additional ligating sites leading to N2S2O2 octahedral complexes.7,8 While the N2S2Ni complexes based on the daco framework have produced an extremely useful body of work, the synthesis of the daco diazacycle is not trivial. Hence we explored the use of the commercially available diazacycloheptane (dach) as a platform for further N2S2 ligand development.9 In addition, the (1) Farmer, P. J.; Reibenspies, J. H.; Lindahl, P. A.; Darensbourg, M. Y. J. Am. Chem. Soc. 1993, 115, 4665. (2) Musie, G.; Farmer, P. J.; Tuntulani, T.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1996, 35, 2176. (3) Grapperhaus, C. A.; Darensbourg, M. Y. Acc. Chem. Res. 1998, 31, 451, and references therein. (4) Bellefeuille, J. A.; Grapperhaus, C. A.; Derecskei-Kovacs, A.; Reibenspies, J. H.; Darensbourg; M. Y. Inorg. Chim. Acta 2000, 300, 73. (5) Maroney, M. J.; Choudhurry, S. B.; Bryngelson, P. A.; Mirza, S. A.; Sherrod, M. J. Inorg. Chem. 1996, 35, 1073. (6) Ashby, M. T.; Enemark, J. H.; Lichtenberger, D. L. Inorg. Chem. 1988, 27, 191. (7) Smee, J. J.; Goodman, D. C.; Reibenspies, J. H.; Darensbourg, M. Y. Eur. J. Inorg. Chem. 1999, 539. (8) Goodman, D. C.; Tuntulani, T.; Farmer, P. J.; Darensbourg, M. Y.; Reibenspies, J. H. Angew. Chem., Int. Ed. Engl. 1993, 105, 72. (9) Grapperhaus, C. A.; Bellefeuille, J. A.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1999, 38, 3698.

dach diazacycle would permit an assessment of decreasing scaffold ring size on the formation of the N2S2 ligand and the physicochemical properties of the resulting nickel(II) complexes. An important difference was found in the reactivity of dach vs daco with episulfides to yield the mixed N/S ligands (Scheme 1). Two equivalents of episulfide, ethylene sulfide, or isobutylene sulfide readily add to the daco diamine, yielding N,N′-bis(mercaptoethyl)-1,5-diazacyclooctane (H2bmedaco) and N,N′bis(2-mercapto-2-methylpropyl)-1,5-diazacyclooctane (H2bmmpdaco), respectively. In contrast, under similar conditions, only one equivalent of episulfide will add to the dach diamine and forms N2S tridentate ligands. With significantly longer reaction times, 2 equiv of the less sterically hindered ethylene sulfide adds, producing H2bmedach. In this paper we present the X-ray crystal structures and chemical properties of the (bmedach)Ni and the S-templated macrocyclic (propyl-bmedach)NiBr2 com-

10.1021/ic0008619 CCC: $20.00 © 2001 American Chemical Society Published on Web 06/06/2001

3602 Inorganic Chemistry, Vol. 40, No. 14, 2001 plexes. The structural features of these compounds are discussed in relation to their magnetic, spectroscopic, and electrochemical properties; these properties are then compared to those of the analogous daco derivatives. Experimental Section General Methods. Nickel acetylacetonate (Strem Chemical Company), 1,3-dibromopropane (Lancaster Synthesis Ltd.), 1,4-diazacycloheptane, ethylene sulfide, and methyl iodide (Aldrich Chemical Co.) were used as received. Benzene, methanol, toluene, and diethyl ether were dried according to standard procedures and freshly distilled prior to use.11 NMR solvents were purchased from Cambridge Isotopes Laboratories and used as received. The ligand H2bmedach formed disulfide species upon long (hours) exposure to O2, necessitating that the syntheses of the ligand and the (bmedach)Ni complex be carried out using dry and degassed solvents under dry N2 or Ar atmosphere. Once isolated, the (bmedach)Ni showed little sensitivity toward O2 or water. Due to the hygroscopic nature of the alkylated derivatives of (bmedach)Ni, their syntheses required careful exclusion of water. Physical Methods. Proton NMR spectra were recorded in d4-MeOH on a Varian XL-200 FT-NMR. Mass spectral analyses were done at the Laboratory for Biological Mass Spectroscopy at Texas A&M University. Positive ion fast atom bombardment mass spectra were recorded in a 4-nitrobenzyl alcohol (NBA) matrix using a VG-70S spectrometer with a xenon source having a particle energy of 10 keV. The data were collected by a VG11-250J data system. UV-vis spectra were measured with a Hewlett-Packard 8453 diode array spectrophotometer using quartz cells (1.00 cm path length). Elemental analyses were performed by Canadian Microanalytical Services, Ltd., Delta, British Columbia, Canada. Cyclic voltammograms were recorded on a BAS-100A electrochemical analyzer with a glassy carbon working electrode, a platinum wire auxiliary electrode, and a Ag0/AgNO3 reference electrode. Samples (ca. 2.5 mM) were measured in CH3CN with tetra-n-butylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte. Potentials were standardized with Cp2Fe+/Cp2Fe (+400 mV vs NHE). It should be noted that the preferred reference potential for ferrrocene in acetonitrile solutions is +600 mV.12 However, to facilitate comparisons with complexes previously characterized in our laboratory, we will continue to use the +400 mV reference. X-ray Crystal Structure Analysis. The X-ray crystal structures were solved at the Crystal Structure Laboratory Center for Chemical Characterization and Analysis at Texas A&M University. Cell parameters and data collection summaries for (bmedach)Ni and (propylbmedach)NiBr2 are given in Table 1. The X-ray crystal data of (bmedach)Ni were obtained on a Siemens P-4 single-crystal X-ray diffractometer operating at 50 kV and 30 mA, Mo KR (λ ) 0.71073 Å). The X-ray crystal data of (propyl-bmedach)NiBr2 were obtained on a Bruker SMART-1000 single-crystal X-ray diffractometer operating at 50 kV and 40 mA, Mo KR (λ ) 0.71073 Å) radiation. For both complexes, data reduction was done with SAINTPLUS (Bruker);13 structure solution, with SHELXS-86 (Sheldrick);14 and structure refinement, with SHELXL-97 (Sheldrick).15 Both crystal structures were solved by direct methods, and anisotropic refinement for all nonhydrogen atoms was done by a full-matrix least-squares method. Syntheses. The (bmedaco)Ni16 and its corresponding (propylbmedaco)NiBr2 bicyclic complex17 were prepared according to published procedures that served as the guide for the syntheses of the analogous dach complexes that follow. (10) Ashby, M. T.; Alguindigue, S. S.; Khan, M. A. Inorg. Chim Acta 1998, 270, 227. (11) Gordon, A. J.; Ford, R. A. The Chemist’s Companion; Wiley and Sons: New York, 1972; pp 429-436. (12) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chim. Acta 2000, 298, 97. (13) Bruker. SAINT-Plus, version 6.02; 1999, Madison, WI. (14) Sheldrick, G. SHELXS-86, Program for Crystal Structure Solution; Institu¨t fu¨r Anorganische Chemie der Universita¨t: Tammanstrasse 4, D-3400, Go¨ttingen, Germany, 1986. (15) Sheldrick, G. SHELXL-97, Program for Crystal Structure Refinement; Institu¨t fu¨r Anorganische Chemie der Universita¨t: Tammanstrasse 4, D-3400, Go¨ttingen, Germany, 1997.

Smee et al. Table 1. Experimental Data for the X-ray Crystal Structures of (bmedach)Ni and (propyl-bmedach)NiBr2 chemical formula fw space group a, Å B, Å C, Å β, deg V, Å3 F, g/cm3 Ζ temp, K radiation (λ, Å) µ, mm-1 R(F)a Rw(F2)a,b

(bmedach)Ni

(propyl-bmedach)NiBr2

C9H18N2S2Ni 277.08 orthorhombic Pnma 10.056(4) 14.796(6) 7.526(2) 90.000 1119.8(7) 1.644 4 193 Mo KR (0.71073) 2.068 0.0576 0.1478

C12H22Br2N2NiS2‚H2O 494.98 monoclinic P21/c 14.0180(9) 10.4274(6) 13.3327(8) 113.1340(10) 1792.15(19) 1.835 4 110 Mo KR (0.71073) 5.772 0.0593 0.1449

a Residuals: R ) ∑|Fo - Fc|/∑Fo; Rw(F2) ) {[∑w(Fo - Fc)2]/ [∑w(Fo)2]}1/2. b I > 2σ(I).

N,N′-Bis(2-mercaptoethyl)-1,4-diazacycloheptanenickel(II), (bmedach)Ni. In a 250 mL Schlenk flask equipped with a magnetic stir bar, 5.24 g of 1,4-diazacycloheptane, dach (52.4 mmol), were dissolved in 100 mL of dry benzene. The mixture was heated to 65 °C under Ar, after which time 8 mL (134 mmol) of ethylene sulfide was added. The mixture was stirred for 72 h at 65 °C. A fine white powder that precipitated from solution was removed by anaerobic filtration through Celite in a glass-fritted funnel. The solvent was removed from the filtrate in vacuo while maintaining the temperature at 60-65 °C; continued heating under vacuum removed any unreacted dach. The resulting pale yellow to colorless H2bmedach liquid was transferred to a 500 mL Schlenk flask and dissolved in 50 mL of dry toluene. To this was added, dropwise via cannula, a solution containing an equimolar amount (relative to that of the dach diamine) of Ni(acac)2 dissolved in 250 mL of dry toluene. During this time a tan-colored precipitate formed. Upon complete addition of the Ni(acac)2, the mixture was stirred for an additional hour and the crude (bmedach)Ni precipitate was collected by filtration (anaerobic conditions are not required). The complex was purified by recrystallization from a hot (60 °C) 50:50 MeOH/H2O solvent mixture. Yield: 1.53 g (10.5% based on dach). UV-vis spectrum in MeOH [λmax, (, M-1 cm-1)]: 457(290), 278(12500), 236(11500). Mass spectrum (m/z) ) 277. Anal. Calcd (found): C, 39.02 (39.08); H, 6.55 (6.47); N, 10.11 (10.13). 4-8-Dithia-1,11-diazabicyclo[9.3.2]hexadecanenickel(II) bromide, (propyl-bmedach)NiBr2. To a 25 mL CH3CN slurry of 110 mg (0.40 mmol) of (bmedach)Ni was added 0.5 mL (4.1 mmol) of 1,3dibromopropane. The solution changed from a light yellow to redorange as the (bmedach)Ni was consumed. The few crystals that formed overnight from the stirred solution were augmented by precipitation upon addition of diethyl ether. The liquid was decanted and the solid washed with an additional 2 × 10 mL of diethyl ether; the orange powder was dried overnight in vacuo. Yield: 103 mg (53.7%). UVvis spectrum in MeCN [λmax, (, M-1 cm-1)]: 495(320), 271(9900), 218sh(15500), 203(17800). Molar conductivity measured in CH3CN: 320 S/cm‚mol. Mass spectrum (m/z) ) 318. Anal. Calcd (found): C, 30.1 (29.1); H, 5.05 (5.02); N, 5.85 (5.61). 1,4-Bis[2-(methylsulfanyl)ethyl]-1,4-diazacycloheptanenickel(II) Iodide, (Me2-bmedach)NiI2. To a 50 mL CH3CN slurry of 190 mg (0.69 mmol) of (bmedach)Ni was added 3 equiv (135 µL, 2.2 mmol) of MeI via syringe. The solution was stirred overnight, during which time black crystals formed on the bottom of the flask. Addition of a large excess of ether precipitated the remainder of the product as a brown solid, which was collected by filtration (aerobic) and dried in vacuo. The product thus obtained was used without further purification (16) Mills, D. K.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1990, 29, 4364. (17) Musie, G.; Reibenspies, J. H.; Darensbourg, M. Y. Inorg. Chem. 1998, 37, 302.

Subtle Bite-Angle Influences on N2S2Ni Complexes

Inorganic Chemistry, Vol. 40, No. 14, 2001 3603 Table 2. Comparison of Metric Parameters from the dach and dacoa Derivatives

Figure 1. Thermal ellipsoid plot (at 50% probability) of (bmedach)Ni. The hydrogen atoms are omitted.

Figure 2. Thermal ellipsoid plot (at 50% probability) of the monocationic unit of (propyl-bmedach)NiBr2. The noninteracting bromide counterion, the single water of crystallization, and all hydrogen atoms are omitted. and was stored in a desiccator prior to use. Yield: 239 mg (62.1%). Anal. Calcd (found): C, 23.6 (22.9); H, 4.31 (4.34); N, 4.99 (5.67).

Scheme 2

Results and Discussion Preparation and Structures of the dach Complexes. Summarized in Scheme 2 are the details for the syntheses of (bmedach)Ni and (propyl-bmedach)NiBr2. Both products were crystalline and suitable for X-ray diffraction studies; details of the collection and refinement are presented in Table 1, and molecular structures are given in Figures 1 and 2. Listings of selected metric data and structural features are given in Table 2 for the new compounds of this study and for the diazacyclooctane analogues, (bmedaco)Ni and (propyl-bmedaco)NiBr2.16,17 The N2S2 donor ligands of (bmedach)Ni and (propyl-bmedach)NiBr2 maintain square planarity

a The metric parameters for the daco derivative, given in italics, are taken from refs 16 and 17. b Defined as the nickel displacement from the best N2S2 plane. c Defined as the intersection of the normals of the S-X-S and N-X-N planes where X is the centroid of the N2S2 plane. As the nickel atom is not necessarily equidistant from the nitrogen and sulfur atoms, the centroid rather than the nickel is used to define the planes. d As viewed across the N2S2Ni plane.

in both thiolate and thioether forms. The structures shown in Table 2 illustrate that the chair conformation predominates in the six-membered nickel-diazacyclohexane and nickel-dithiacyclohexane rings. For the (bmedaco)Ni complex, there is a pronounced Td twist distortion from square planarity of ca.

3604 Inorganic Chemistry, Vol. 40, No. 14, 2001 Table 3. Comparison of Electronic Spectroscopya and Electrochemistrya of the dach and dacob Derivatives

colorc λmax (nm) () E1/2 NiII/I Epa E1/2, rev

(bmedach)Ni (bmedaco)Ni

(propyl-bmedach)NiBr2 (propyl-bmedaco)NiBr2

red-brown/yellow-brown purple/purple 457(290)d 506(640) 602 sh -2051 mV -2048 mV +254 mV +246 mV +756 mV +801 mV

orange/orange-red purple/Violet 495(320) 410(151) 581(142) -813 mV -670 mV +882 mV +869 mV +1138 mV +1108 mV

a Unless specified otherwise, all results were obtained from acetonitrile solutions. b The parameters for the daco derivatives are given in italics and are taken from refs 16 and 17. c In the solid state/solution (acetonitrile or methanol). d The electronic spectrum of (bmedach)Ni was measured in methanol.

13.3°.16 In contrast the (bmedach)Ni complex is essentially square planar, which accounts for sharper signals in the 1H NMR spectrum as compared to the (bmedaco)Ni complex. Whereas the differences in the Ni-N and Ni-S bond distances are of little statistical significance, the Ni-N and the Ni-S distances in the dach derivatives are shorter than those in the daco derivatives. Larger metric differences are in the ∠N-Ni-N angles enforced by the two diazacycles, which are observed at 83° and 89° for the dach and daco derivatives, respectively. As a result, the ∠S-Ni-S angles open to 95° and 94° for (bmedach)Ni and (propyl-bmedach)NiBr2, respectively; in contrast, both daco derivatives have ∠S-Ni-S angles of 89°. In the (propyl-bmedach)NiBr2 and (propyl-bmedaco)NiBr2 analogues, a single bromide is positioned to interact with nickel(II) in an axial fashion, generating a quasi-squarepyramidal coordination geometry. The comparative data of Table 2 show this to be a long-range Br-‚‚‚NiII interaction, ca. 2.93.0 Å. A bona fide Ni-Br bond in a similar N2S2NiBr complex is 2.45 Å.18 Also supporting this description is the fact that the nickel atoms in both the (propyl-bmedach)NiBr2 and (propylbmedaco)NiBr2 complexes are slightly pulled out of the bestfit N2S2 plane and oriented toward Br-. This pseudo-squarepyramidal geometry accounts for the observed paramagnetism, vide infra. A cocrystallized water molecule appears to be H-bonded to both Br- counterions, as seen by the O‚‚‚Br distances of 3.46 and 3.37 Å for the Br- interacting with the Ni center and the noninteracting Br-, respectively. Electronic Properties. The daco dithiolate derivative is more richly colored than its dach analogue; the UV-vis solution spectra and molar absorptivity coefficients in CH3CN reflect these differences, suggesting that the distortions seen in the solid-state structures are maintained in the solution samples (Table 3). The solution spectroscopic properties of the derivatives (propyl-bmedaco)NiBr2 and (propyl-bmedach)NiBr2 are more similar, most likely on account of the restrictions in the N2S2 macrocyclic plane, as well as the tendency for axial interactions of anion or solvent for the dicationic complexes, which prevent tetrahedral distortions. Since the conductivity of (propyl-bmedach)NiBr2 in CH3CN is consistent with a unidielectrolyte,19 a replacement of the interacting Br- by CH3CN or adventitious water, freeing both Br- counterions, is suggested. (18) Buonomo, R. M.; Reibenspies, J. H.; Darensbourg, M. Y. Chem. Ber. 1996, 129, 779. (19) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81.

Smee et al. It should be noted that the 1H NMR spectra of both propyl macrocycles display somewhat broadened peaks, with the daco derivative significantly more broadened than its dach analogue. While broadened peaks are consistent with a thermally accessible paramagnetic state resulting from pentacoordination, variable-temperature 1H NMR spectra showed no changes in line widths for either complex. Electrochemical Properties. A summary and comparison of the electrochemical properties of (bmedach)Ni, (bmedaco)Ni, (propyl-bmedach)NiBr2, and (propyl-bmedaco)NiBr2 are given in Table 3. The four complexes all show reversible waves assigned to the NiII/I redox couple. Irreversible anodic events are ascribed to thiolate oxidation in the case of the dithiolate derivatives and bromide counterion oxidation for the salts of the macrocyclic derivatives. A quasi-reversible wave is observed for all four complexes centered in the range of +700 to +1200 mV. A NiIII/II redox couple is expected to be in this region; however the species that gives rise to this couple has not been defined. There is no difference in the NiII/I redox potential for (bmedach)Ni and (bmedaco)Ni; however the (propyl-bmedaco)NiBr2 derivative displays a 140 mV stabilization of the NiI oxidation state relative to (propyl-bmedach)NiBr2. This observation could be attributed to the smaller cavity size of the (propylbmedach)NiBr2 macrocycle or to a decreased NiII-axial ligand or specific counterion interaction in (propyl-bmedaco)NiBr2. Since conductivity measurements indicate that the latter is not the case, the decreased N2S2 cavity size and accompanying decreased flexibility would appear to be the more likely explanation. In support of the cavity-size/flexibility argument, the open-chain dithioether derived from double MeI alkylation of (bmedach)Ni, (Me2-bmedach)NiI2, shows the most accessible NiII/I couple (-712 mV) of the three dach derivatives, while the daco analogue, (Me2-bmedaco)NiI2, with its larger cavity size, has a NiII/I redox couple at -482 mV.16 This lowest of all values is consistent with the greater flexibility of the open-chain tetradentate, dithioether ligand. Summary and Remarks The effects of ring size, and of fused rings, in doubly bridged tertiary N donors in open-chain ligands on complex stability have been extensively explored and recently reviewed by Martell and Hancock.20 Pertinent to the subject of this paper are references to square-planar nickel(II) complexes of the following ligands:

Both comparisons of dach versus daco frameworks find the latter to form more stable complexes with nickel(II), a result that is ascribed to a more favorable N-Ni size match or orbital orientation match for daco. On the basis of qualitative results (ease of isolation and stability on storage) we see no difference in the stability of the dithiolate and macrocyclic dithioether complexes thus far derived from daco versus dach. According to the analysis of Hancock et al.,20 the ideal geometry for a metallodiazacyclopentane ring should have a

Subtle Bite-Angle Influences on N2S2Ni Complexes

Inorganic Chemistry, Vol. 40, No. 14, 2001 3605

N-M distance of 2.5 Å and a N-M-N angle of 69°; for a metallodiazacyclohexane ring the optimal parameters are 1.6 Å and 109.5°, respectively. Neither of these metric sets is congruent with the data presented here. The Ni-N and Ni-S distances vary by less than 0.1 Å. While the (bmedaco)Ni complex shows regular 90° angles in the N2S2 plane, and a significant Td twist distortion, the dach diamine imparts a narrowing of the N-Ni-N angle to 83° and an expansion of the S-Ni-S to 95°, keeping a more regular square plane. These changes evidently compensate and place the NiII in an environment equally conducive to the acceptance of an electron to form a NiI species. While the angle data are consistent with the Hancock prediction, the trend in the distance data is the opposite, with the Ni-N and Ni-S distances being shorter in the dach derivatives. That is, the N2S2 cavity is slightly smaller in the open-chain bmedach(Ni) as well as in the macrocyclic (propylbmedach)NiBr2 as compared to the daco analogues. The principal difference in the physical properties between the complexes lies in the ease or accessibility of the NiII/I redox couple of the N2S2 dithioether macrocycle complexes (propylbmedaco)NiBr2 and (propyl-bmedach)NiBr2. The 140 mV stabilization of the former relative to the latter may be attributed to a greater flexibility of the larger cavity to accommodate the increased size and geometry preference of NiI. As noted above, values for the NiII/I couple of the dimethylated complexes (Me2bmedaco)NiI2 and (Me2-bmedach)NiI2 show an even more dramatic difference. The redox couples in both open-chain, dithioether complexes are more easily accessed than those of

the more rigid closed-chain macrocycles, with the daco derivatives again providing more stabilization of the NiI oxidation state than those of the dach. We conclude that the (bmedach)Ni complexes are minimally different from those derived from (bmedaco)Ni, and the rich S-based reactivity found for the latter should be available to the former. Clearly the use of (bmedach)Ni as a templating reagent in the preparation of the S-S linked macrocycle is not impaired by the N2S2 angle differences. Preliminary studies of oxygen reactivity find the (bmedach)Ni complex to be less reactive with ground-state O2 than is (bmedaco)Ni. Nevertheless with limited amounts of H2O2, a sulfone, the [N,N′-(mercaptoethyl)(sulfinatoethyl)diazacycloheptane]Ni(II), has been obtained. Both constrained N2S2 frameworks do not show the extensive S-oxygenation that has been reported for a more flexible ethylenediamine-based N2S2Ni complex.21

(20) Martell, A. E.; Hancock, R. D. Metal Complexes in Aqueous Solutions; Plenum Press: New York, 1996; Chapter 3.

(21) Henderson, R. K.; Bouwman, E.; Spek, A. L.; Reedijk, J. Inorg. Chem. 1997, 36, 4616.

Acknowledgment. We gratefully acknowledge financial support from the National Science Foundation (CHE-9812355 for this work, and CHE 85-13273 for the X-ray diffractometer and crystallographic computing system) and contributions from the R. A. Welch Foundation. Supporting Information Available: Tables of crystallographic data, collection parameters, atomic coordinates, packing diagrams, a complete listing of bond lengths and bond angles, and the cyclic voltammograms for (bmedach)Ni and (propyl-bmedach)NiBr2; also included is the cyclic voltammogram of (Me2-bmedach)NiI2. This material is available free of charge via the Internet at http://pubs.acs.org. IC0008619